Alkali metal cyanoborohydride reductive bleaching liquor

ABSTRACT

The invention provides an aqueous reductive bleaching liquor containing the material to be bleached selected from groundwood pulp, kraft pulp, chemigroundwood pulp and clay dispersed therein in conventional amounts. As the reducing agent, a cyanoborohydride is dissolved in the liquor in an amount between about 0.01 and about 0.3 percent based upon the dry weight of the selected material to be bleached. The liquor has a pH between about 3 and 6 and is stable at a temperature between room temperature and about 180* F. The cyanoborohydride is selected from lithium, potassium, and sodium cyanoborohydride, sodium cyanoborohydride being preferred.

United States Patent Wade [4 1 Jan. 25, 1972 [54] ALKALI METAL CYANOIOROHYDRIDE REDUCTIVE BLEACHING LIQUOR [72] Inventor: Robert C. Wade, Ipswich, Mass.

[73] Assignee: Ventren Corporation, Beverly, Mass.

[22] Filed: Aug. 19, 1969 2] Appl. No.: as1,472

[52] 11.8. CI ..252/l88, 8/107, 23/358, 162/80, 252/105 [51] Int.Cl. ..D2lc 9/10, D061 3/10 [58] Field of Search ..252/188, 105; 23/358; 8/107, 8/101; 162/80, 81

[5 6] References Cited UNITED STATES PATENTS 2,992,885 7/1961 Jackson et al ..23/358 3,179,639 4/1965 Hutchinson ..260/83.3

3,318,657 5/1967 Wade ..8/l07 Primary Examiner-Richard D. Lovering Assistant Examinerlrwin Gluck Attorney-J. Harold Boss [5 7] ABSTRACT The invention provides an aqueous reductive bleaching liquor containing the material to be bleached selected from groundwood pulp, kraft pulp, chemigroundwood pulp and clay dispersed therein in conventional amounts. As the reducing agent, a cyanoborohydride is dissolved in the liquor in an amount between about 0.01 and about 0.3 percent based upon the dry weight of the selected material to be bleached. The liquor has a pH between about 3 and 6 and is stable at a temperature between room temperature and about 180 F. The cyanoborohydride is selected from lithium, potassium, and sodium cyanoborohydride, sodium cyanoborohydride being preferred.

6 Claims, Na Drawings ALKALI METAL CYANOBOROHYDRIDE REDUCTIVE BLIEACHING LIQUOR The invention relates to the reductive bleaching of groundwood pulp, kraft pulp, chemigroundwood pulp, and clay.

The reductive bleaching of groundwood pulp and kraft pulp which contains --5 percent or more of residual liquor as well as clay has long been practices. Sodium and zinc hydrosulfite have been the bleaching agents of choice. These materials have certain disadvantages, however, which cause a severe problem to pulp, paper and clay producers. The sulfur residues from the bleaching operation are acidic and cause problems of corrosion of equipment and the Fourdrinier wire. Appreciable amounts of hydrogen sulfide can be found in many pulp or paper mill waters which come from the hydrosulfite bleaching operation. Zinc hydrosulfite is generally preferred because of its greater stability than sodium hydrosulfite in the preferred bleaching pH range, namely, pH of 4 to 6. However, the latter material has the problem of contributing to stream pollution in the form of zinc ions which are know to be highly toxic to fish life.

Attempts to use the alkali metal borohydrides as reductive bleaching agents for these pulps and clay have not been successful because of the rapid hydrolysis of these borohydrides at a pH below 9. While some bleaching can be accomplished with these borohydrides, inordinately large and uneconomical amounts of these borohydrides (5 to 20 pounds per ton of dry pulp or clay) are required to accomplish the desired bleaching.

l have discovered that the above problems and difficulties can be overcome by the use of an alkali metal cyanoborohydride as the reducing agent and that small and economical amounts effectively bleach liquor-containing wood pulps, such as groundwood, chemigroundwood or kraft pulps, and clay. For reasons of economics, I prefer to use sodium cyanoborohydride (NaBH CN). The conventional method and equipment used when the reducing agent is hydrosulfite is used when a cyanoborohydride is used as the reducing agent, except that a lower pH of between about 3 to 6 may be used and a smaller quantity of the cyanoborohydride is required. No corrosive residues remain after the bleaching operation is complete.

Thus the invention provides an aqueous reductive bleaching liquor containing the material to be bleached dispersed therein and an amount of cyanoborohydride dissolved therein between about 0.0l and about 0.3 percent by weight based upon the dry weight of the selected material to be bleached. This aqueous reductive bleaching liquor has a pH between about 3 and 6 and is stable at a temperature between room temperature and about 180 F.

Sodium and lithium cyanoborohydride may be economically synthesized by a process described in my copending patent application Ser. No. 833,722, filed June 16, 1969. The method comprises mixing substantially anhydrous hydrogen cyanide with substantially anhydrous lithium or sodium borohydride at a temperature between C. and 100 C. in a substantially anhydrous solvent, such as tetrahydrofuran. The process for preparing potassium cyanoborohydride is described in my copending U.S. Pat. application Ser. No. 833,766, filed June 16, 1969, the process being similar to that for preparing the sodium and lithium cyanoborohydrides, except that dimethyl formamide must be used as the solvent.

The invention is illustrated further by the following specific examples.

EXAMPLE 1 The groundwood pulp used for these experiments was a southern pine. The consistency was adjusted to 3 percent. All tests were run on 10 grams of oven dry pulp. Bleaching was 2 carried out in polyethylene bags in a constant temperature bath at F. for 1 hour at a pH of 4.0. Then, a pulp sheet was prepared and dried by standard procedures. Brightness measurements were made on a photovolt reflectance meter.

1: NaBH=CN Brightness (on wt. of CD. pulp) Brightness Gain Control 61.3 0.25 67.0 5.2 0.095 65.8 4.l 0.031 63.9 2.2

EXAMPLE 2 Kraft liner board pulp was bleached at pH =4, I40 F. for [5 minutes. Results are summarized below:

b NaBH CN Brightness (on wt. of CD. pulp) Brightness Gain Control l8.0 0.25 22.8 4.8 0.09 2L0 3.0

There was a marked shift in color from red to yellow in these pulps.

EXAMPLE 3 NaBH CN Brightness (based on wt. dry clay) Brightness Gain Control 74.0 0.05 76.0 2 0.1 78.0 4 0.2 8 L5 7.5

I claim:

1. An aqueous reductive bleaching liquor comprising a material selected from the group consisting of groundwood pulp, kraft pulp, chemigroundwood pulp and finely divided clay dispersed therein, said liquor having an amount of a cyanoborohydride between about 0.01 and about 0.3 percent by weight based upon the dry weight of said selected material dissolved therein and having a pH between about 3 and 6 and being stable at a temperature between room temperature and about F., said cyanoborohydride being selected from the group consisting of lithium, sodium, and potassium cyanoborohydrides.

2. An aqueous reductive bleaching liquor as claimed by claim 1 wherein said cyanoborohydride is sodium cyanoborohydride.

3. An aqueous reductive bleaching liquor as claimed by claim 2 wherein said material is groundwood pulp.

4. An aqueous reductive bleaching liquor as claimed by claim 2 wherein said material is kraft pulp.

5. An aqueous reductive bleaching liquor as claimed by claim 2 wherein said material is chemigroundwood pulp.

6. An aqueous reductive bleaching liquor as claimed by claim 2 wherein said material is clay. 

2. An aqueous reductive bleaching liquor as claimed by claim 1 wherein said cyanoborOhydride is sodium cyanoborohydride.
 3. An aqueous reductive bleaching liquor as claimed by claim 2 wherein said material is groundwood pulp.
 4. An aqueous reductive bleaching liquor as claimed by claim 2 wherein said material is kraft pulp.
 5. An aqueous reductive bleaching liquor as claimed by claim 2 wherein said material is chemigroundwood pulp.
 6. An aqueous reductive bleaching liquor as claimed by claim 2 wherein said material is clay. 